Transrectal ultrasonography of follicular dynamics and early pregnancy in mares: an on-site visual reference

Abstract

This study aimed to characterize ovarian follicular dynamics and to describe the ultrasonographic features of early to after-gestation in mares under extensive field conditions. A total of 100 clinically healthy Spanish-type mares, averaging 8 ± 0.7 years of age and 500 ± 50 kg in body weight, were monitored over a 730-day period in three rural locations in Tamaulipas, Mexico. Transrectal ultrasonography was performed daily across a full estrous cycle (~21 days) to assess follicular development in non-pregnant mares. For mares confirmed pregnant 15 days post-mating, five ultrasound evaluations were conducted between days 15 and 150 of gestation. A total of 500 follicular and 500 gestational ultrasonographic images were obtained. Follicles were classified into three groups by diameter: 8–20 mm (n = 250; 50.0%), 21–30 mm (n = 150; 30.0%), and 32–45 mm (n = 100; 20.0%), with mean diameters of 13.7 mm, 25.4 mm, and 39.7 mm, respectively. The mean number of follicles per mare was 5.0, reflecting typical follicular activity during one estrous cycle under tropical field conditions. Gestational scans were distributed as follows: days 15–28 (n = 167; 33.4%; mean age: 21.9 d), days 30–45 (n = 166; 33.2%; mean age: 38.8 d), and days 50–150 (n = 167; 33.4%; mean age: 75.1 d). Key gestational features, such as the embryonic vesicle, heartbeat, and fetal skeleton, were documented per stage. In conclusion, structured ultrasonographic benchmarks for assessing follicular and gestational stages in mares were provided, offering practical tools for reproductive monitoring in field conditions.